We have the same exact implementation as mentioned in the SO post. There are no errors when using the follow callback, it's just that on mobile, the callback never occurs. On desktop, it works perfectly fine.
When clicking the follow button on mobile, it opens in a new tab. In comparison, when clicking the tweet button on mobile it opens in a popup that closes after a successful tweet and we receive a response back for the callback.
I'm going to assume that the fact that the follow button is opening in a new tab on mobile is what is causing the issue.